The 20-year-old required medical attention on the pitch on Saturday before being airlifted to hospital.
The game was abandoned, with Ajax tweeting soon after that Nouri had heart arrhythmias, a condition whereby the organ beats either too fast or too slow.
A later statement said the reasons for his collapse were unclear and that he was being kept in intensive care but "out of danger".
"Since arriving at the hospital he has been thoroughly examined. He had a weak heart beat and is being kept asleep to allow for optimal treatment and care," the club added.
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Nouri made 15 appearances for the Dutch club last season, including three in the Europa League, though he did not play in the final, a defeat to Manchester United.
He is a Dutch youth international having been born in Amsterdam to parents of Moroccan descent.
The incident comes a little over a month after Ivory Coast international Cheick Tiote collapsed in training with Chinese club Beijing Enterprises and died on June 5.
(Reporting by Mark Gleeson; Additional reporting by Nick Said; Editing by Amlan Chakraborty)
